Wood chips being one of the main raw materials entering into pulp production processes such as thermomechanical pulping (TMP), chemical-thermomecanical pulping (TCMP) or mechanical pulping (MP) processes, or into production processes of other wood-based products such as fibreboards (MDF, HDF) variations in their physical properties have a direct impact on process control performance as well as on final product qualities. In the particular case of a TMP process, the quality of wood chips being fed to the refiners is of a great importance, since it is known to affect the refining process. It is well known that a typical TMP process is characterized by three critical operational variables, namely specific energy, production rate and consistency. For a given process design, specific energy consumption is the parameter that correlates most strongly to evolving pulp properties, as explained by Mosbye, K., et al. in “Use of Refining Zone Temperature Measurements for Refiner Control”, Proceedings of IMPC 2001, Helsinki, Finland, June 2001. While specific energy can in theory be kept constant through adjustments to motor load or production rate, in practice the absence of on-line data about dry wood chip/fibre volume and moisture content means that the control of this variable will be subjected to instability, as mentioned by Cluett, W. R., et al. in “Control and Optimization of TMP Refiners”, Pulp & Paper Canada, 96:5 (1995) pp. 31-35. The production rate, which is directly affected by the quantity of dry fibre refined, has a major impact on both energy consumption and pulp properties. The dilution water flow rate depends on chip moisture content and the consistency target as stated by Myllyneva, J. et al. in the above-mentioned reference. Consistency variations during normal operation are at least 4-6% and even higher, as reported by Hill, J., et al. in “On the Control of Chip Refining Systems”, Pulp & Paper Canada, 94:6 (1993), pp. 43-47. Generally, known TMP process control strategies work according to the hypothesis that wood chip qualities are stable. Any variation in chip quality will be considered as a disturbance in process control. In fact, chip quality changes quite rapidly, and known control strategies cannot efficiently eliminate its influence, which prompts fluctuations of the three operational variables of the refining process mentioned above. Wood species variation is an important factor that can negatively impact pulp quality.
A system for measuring optical reflection characteristics of chips such as brightness, along with other important chip properties, such as moisture content, which is commercially known as the Chip Management System (CMS), is described in U.S. Pat. Nos. 6,175,092 B1 and 7,292,949 B2 (US 2005/0027482) both issued to the present assignee. Some pulp mills have used such system to manage their chip piles according to chip quality, as discussed by Ding et al. in “Economizing the Bleaching Agent Consumption by Controlling Wood Chip Brightness” Control System 2002 Proceedings, Jun. 3-5, 2002, Stockholm, Sweden, pp. 205-209. Chip quality assessment can be defined as the synthesis of measurements made of chip physical characteristics, as explained by Ding et al. in “Effects of Some Wood Chip Properties on Pulp Qualities” 89th Pulp and Paper Annual Conference Proceedings, Jan. 29, 2003, p. 35. Ultimately, this definition depends on the importance of each chip characteristic for a given process. Continuous variations in wood basic density and moisture content occurring in chip flow tend to cause variations in refining consistency, which in turn affect pulp uniformity and energy consumption as reported in U.S. Pat. No. 7,292,949 B2 and by Ding et al. in “ Wood Chip Physical Quality Definition and Measurement”, 2003 International Mechanical Pulping Conference, Quebec City, Canada, Jun. 2-5 2003, pp. 367-373 in view of Tyvaïnen “The Influence of Wood Properties on the Quality of TMP Made from Norway Spruce (Picea abies)—Wood from Old-growth Forests, First-thinnings, and Small Chips” 1995 International Mechanical Pulping Conference, pp. 23-34, 1995.
In pulp mills, visual evaluation of wood chip quality is widely used. From the chip color, a specialist can determine the chip species and estimate freshness, bark, rot, and knot contents. A known approach consists of sorting trees according to their species or blend of species prior to wood chips manufacturing, to produce corresponding batches of wood chips presenting desired characteristics associated with these species. Typically, hardwood trees such as poplar, birch and maple are known to generally produce pale wood chips while conifers such as pine, fir and spruce are known to generally yield darker wood chips. In practice, wood chips batches can either be produced from trees of a same species or from a blend of wood chips made from trees of plural species, preferably of a common category, i.e., hardwood trees or conifers, to seek wood chips uniformity.
Many studies have shown that wood species and dry-based density are the dominant factors in pulping performance and pulp quality. The spruce family is the most favorable species for TMP as mentioned by Varhimo, A. et al, in “Raw Materials” in Sundbolm, J. “Mechanical Pulping” Chapter 5, Fapet OY, 66-104 (1999). Although chip aging can be observed from chip brightness, it is only useful for substantially unvaried wood species. When an unknown proportion of wood species is present, more information is needed to provide reliable chip quality assessment. Basic density is one of the most studied wood properties, and it varies substantially between and within various wood species. Basic density is not, however, an independent property but is determined by several characteristics of wood. As also mentioned by Varhimo et al, variations in wood basic density result in pulp quality variations. There is a good correlation between basic density and dry bulk density, the chip flow usually is metered by volume, and dry bulk density variations will cause fluctuations in the production rate, as reported by Dundar, E. et al, in “Decreasing Specific Energy of Thermomechanical Pulps from Reduction of Raw Materials Variability”, September 2009, TAPPI Journal pp. 23-29.
For the purpose of wood dry-based density measurement, a sampling system used to determine moisture content and bulk density is proposed by Preikschat E. in “Measuring the Moisture and Bulk Density of Pulp Wood Chips for Digester Control—with a Correction Algorithm for Frozen Conditions” Energy Conservation through Instrumentation, ISA Capital Cities Control Conference, May 13-15, 1980, pp. 33-38. This sampling system has been suggested for a white liquor flow rate control of a Kraft process. A height measurement apparatus for determining the volume/density of wood chips on a conveyor is also proposed by Beran et al. in U.S. Pat. No. 6,211,470 B1, where the bone dry bulk density can be measured and used for a Kraft process control. Another sampling system to be used to measure bulk density for digester control is proposed by Bäcklund in European Patent no. 738 342 B1. In this system, the moisture content is estimated from information on the chip weight volume and wood basic density. U.S. Pat. No. 6,447,639 B1 to Warren et al teaches a process for controlling a digester using real time measurement of moisture content and species of wood, without density measurement.
For the purpose of wood species identification, some optical testing methods are proposed by Sum, S. T. et al. in “Laser-excited Fluorescence Spectra of Eastern SPF Wood Species—An Optical Technique for Identification and Separation of Wood species”, Wood Sci. Technol., 25, 1991, pp. 405-413., and by Lawrence, A. H. in “Rapid Characterization of Wood Species by Ion Mobility Spectrometry”, Journal of Pulp and Paper Science, 15 (5), 1989, J196-J199, and a chemical vapor analysis is proposed by Fuhr, B. J. IN “On-line Wood Species Sensor”, Paper Age, September-October 2001, pp. 26-29. These known methods have been applied either off-line in laboratory or on-line for monitoring a specific wood species. However, these techniques cannot be used to evaluate a mixture involving more than two wood species. An on-line measurement system such as described in US 2005/0027482 and Ding et al. cited above, can produce data that is useful for identifying the proportion of pure wood species making up a mixture of wood chips, on the basis of optical reflection and moisture measurements made on wood chips. For example, the brightness of Balsam Fir is quite similar to that of Black Spruce, but Fir's moisture content is about 55%, while Spruce moisture content is about 40%. Likewise, although Jack Pine's moisture content is similar to that of Black Spruce, Pine is the darker species of the two. For a mixture of more than two species, it is possible to estimate a breakdown of the species present. US 2005/0027482 teaches the use of an estimation model based on a feed-forward neural network that is built from optical reflection-based measurements, namely R,G,B,H,S,L, and dark chip content (D), along with moisture measurement as input variables, in which chip freshness (ageing) and species are controlled, and the selection of the input variables for the FFNN has been performed using known Principal Component Analysis (PCA) technique from the trials results. The well known Levenberg-Marquardt algorithm has been used to train the model, to provide at an output thereof an indication of wood species composition, usually representing the purity level of a main species forming a chip sample. However, it has been observed that such approach provides an estimation of the proportion of each species within a range of only about ±10%, which is generally insufficient to allow an efficient control over species variation in wood chips fed to the pulping process. Although US 2005/0027482 teaches that chip quality on-line measurement is very useful for stabilizing chip input, and that feedback information can control chip-feeding screws so as to take suitable proportions of chips from different piles or silos, such approach is not efficient to minimize specific energy. The system described in US 2005/0027482 for measuring chip moisture content and wood species does not involve any chip density measurements.
In prior published application no. US 2006/0278353 also naming the present assignee, there is disclosed the use of a measurement system for estimating and controlling relative proportion of wood chips originating from a plurality of sources characterized by various wood species, in a mass of wood chips to be fed to a process for producing pulp, wherein light reflection-related and density-related properties are used as input in a model characterizing a relation between such wood chip properties and species information. This principle allows efficient monitoring of the variation in wood species composition characterizing the wood chips to be processed, for the purpose of stabilizing chip feeding control and optimizing process parameters adjustment. When installed in the chip feeding process, the measurement system generates on-line chip characteristics information that can be used to control the mixture of chips from the different piles in order to stabilize the dry mass of wood chips entering the digester stage of a Kraft process.
As opposed to chemical processes involving digesters, pulp production processes such as TMP, CTMP or MP generally use, upstream the refining process, processing stages of retention, atmospheric presteaming, washing/dewatering, preheating and/or impregnation (CTMP), etc. Similarly, production processes of wood-based products such as MDF or HDF generally use, upstream the refining process, processing stages of retention and preheating/steaming. Due to these upstream processing stages, the known, direct chip feeding control approaches used in chemical processes involving digesters cannot be used to efficiently stabilize the refining process.